Method of producing colored coatings upon magnesium and its alloys



. above 100 C.

Patented July 29, 1941 METHOD OF PRODUCING oonoann COAT- mes pron .MAGNESIUM AND ITS AL- LOYS Herbert K. De Long, Midland, Mich., assignor t The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing. Application August 12, 1938, Serial No. 224,510

I pressure) employed. The aqueous alkaline solu- 9 Claims.

The invention relates tomethods of produc ing coatings upon magnesium and its alloys: It more particularly concerns an improved method of producing a highly decorative, cor osion-resistant, strongly adherent, flexible coating in a wide variety of colors upon magnesium and its alloys.

Heretofore attempts to produce onmagnesium and its alloys decorative coatings in a wide variety of colors and possessing a substantial resistance to corrosion have not been commercially successful.

An object of the invention is to provide a method of producing strongly adherent, corrosion-resistant coatings in a wide variety of colors upon magnesium and its alloys.

Other objects and advantages will appear as the description of the invention proceeds.

According to my invention such coatings are produced by treating the magnesium or magnesium alloy article with an aqueous solution containing a soluble base of an alkali or alkaline earth metal and a dye soluble therein, the treatment being effected under pressure at elevated temperature.

I am aware that it has been proposed to produce uncolored coatings on articles of magnesium and its alloys by subjecting the article to the action of water or water vapor at a temperature In testing such method, however, I have found colored coatings cannot be produced, and moreover such coatings as are obtained are relatively inferior in corrosion resistance coilip'aredto those obtainable by employing alkaline solutions containing a dye according to the invention.

In carrying out my invention the magnesium or magnesium alloy article is first cleaned, as by wire brushing, 'sandblasting, acid pickling, or bufling according to the degree of lustre desired in the ultimate colored coating. The degree of lustre depends upon the smoothness of the metal surface before being subjected to the coating process. For example, sandblasting the metal prepares it to exhibit a relatively dull colored coating, while bumng or polishing the metal to a bright smooth surface results in the article exhibiting a smooth lustrous surface of any desired color after treatment according to the invention.

After having suitably cleaned the article to be coated, it is then subjected tothe action of an aqueous alkaline solution to which has been added a suitable dye in an autoclave at elevated temperature and pressure, the duration of such treatment depending upon the temperature (or tion is madefby dissolving in water from about 2 to 20 per cent or more by weight of an alkali,

such as the carbonate or'hydroxide oi barium, lithium, sodium, or potassium. It ispre erable to use either sodium carbonate or hydroxide in a concentration of about 10 per cent. The temperature to employ is from about 110 C. to 250 C.

. or more, and the pressure should bethat corresponding to the vapor pressure of the solution at such temperature. Such vapor pressure may be from about 20 to 400 lbs. per sq. in. and may be .used advantageously to regulate the temperature of the operation. The duration of the treatment with the alkaline solution may be from 1 to minutes. 7 and above) a relatively short time of treatment (i. e. Lto 5 minutes) suffices 5 minutes being preferred at 200 C., which temperature corresponds to a vapor pressure of about 225 lbs. per sq. in. At lower temperatures (110 C.-150 C.) the treatment should be more prolonged. By the foregoing treatment there is produced upon the article a coating, with which it appears to form an integral part, that is relatively thin, flexible and hard, the coating being dyed throughout with a color depending upon the dye employed.

The .amount of dye to employ should be between about 0.1 and 0.5 per cent or more by weight of the solution, depending upon the shade desired. .In general, the more concentrated the dye in the solution, the deeper will be the shade of color produced. Dyes suitable for the purpose are those which are soluble in the alkaline solution and which do not decompose 1n the-solution at the temperature employed, A wide variety of colors can be produced by themethod, the dye employed being selected'with reference to its solubility and stability as aforementioned.

The following dyes are illustrative of those I suitable for use in the process, the number opposite the dye name being that of the Color Index, 1st edition, January 1924, published by the Society of Dyers and Colourists, Assurance Build- At the higher temperatures (200 C. Y

By the foregoing method coatings having practically any desired color or shade of color can be obtained. The coating is not only highly decorative but also affords excellent protection against corrosive attack and it is resistant to mechanical abrasion. The method also has the advantage of producing homogeneous, coatings having the same color all the way through and, in the event the coating is scratched, it is not seriously marred, because the dye is laid down with the coating substance with which it forms an integral part.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the steps herein disclosed, provided those stated by any of the following claims or their equivalent be employed.

I therefore particularly point out and distinctly claim as my invention:

Dye Color Index No. 1. In a method of producing a colored corro- Eosine Yellow 768 si tant coating upon an article formed of Q inaldine Yellow 801 a metal elected from the class consi ti of Indian Yellow G 14 magnesium d nesium alloys, the step aphthol Yellow s 10 5 which nsists in subjecting the article t h Nitrazine Yellow 40 action a ution consisting of water and a Methyl Violet 0 base l c ed from the class consisting of th Crystal Violet 6B 681 Soluble 8llki'ill and alkaline earth metal bases Methylene Blue 2130 922 and a d oluble in the solution, said step being Aniline Blue 689 10 carriedt nder a pressure of at least 20 Alizarin Blue S 10 7 pounds per quare inch and at a temperature Eosine Bluish 771 above the normal boiling point of the solution. Benzo Blue 23 406 2. In a method of producing a colored corro- Anmin Blue 1066 sion-resistant coating upon an article formed or Fast Acid Black NE]; u 304 a metal e ed from the class consisting of Diazine Black H" Extra n 401 magnesium d gnesium alloys, the step Niagara sky Blue 63 n 518 which Consists in subjecting the article to the Acid Blue-Black 246 action of a solution consisting of water and e, Diamine Black BE 401 base selected from the class consisting of the Orange II 151 soluble alkali and alkaline earth metal bases Orange Gd 27 and a d e soluble in the solution, said step be- Methyl 0mg; 142 t carried t under a pressure of at least 20 n m Y 20% 3;E;'::":': 1040 pounds per square inch and at a temperature Fluorescein 766 above 110 c, Re u A 724 3. A method according to claim 2 wherein the Bismark Brown 331 base is an alkali metal hydroxide. Phenolphthalein 764 A method according t claim 2 wherein the Acid Fuchsine 30 base is n a kali metal carbonate, Azo Fuchsine 61; n 154 In 8 m thod of producing a colored corro- Rhodamine n 749 30 i a t coating upon an article formed or Xylene Cyano1 F1 n 715 a metal ected from the class consisting of Cochineal 1239 magnesium nd magnesium alloys, the step Alizarin cyeiiii'ie 1050 which nsi in subjecting the article to the Bordeaux B 88 action f a solution consistin of water and e- Safranine 6B u 843 tWeen 2 and 20 per cent by weight of a base ianaiuehsine 6E 54 g g a s g onsist i n g bof the srcllluthle u a a an a a eear me ase a fgf g (Butter Yel- 19 tween 0.1 and 0.5 per cent by weight of a soluble dye, at a temperature between about C. and about 250 C., and under a pressure corresponding to the vapor pressure of the solution at the operating temperature, said pressure being between about 20 and about 400 pounds per square inch.

6. A method according to claim 5 wherein the base is sodium hydroxide.

7. A method according to claim 5 wherein the base is lithium hydroxide.

8. A method according to claim 5 wherein the base is sodium carbonate.

9. An article formed of a metal selected from the class consisting of magnesium and mag nesium alloys and having bonded integrally thereto an adherent corrosion-resistant homogeneously colored coating producing according to the method of claim 1.

HERBERT K. DE LONG. 

